Chip resistor and method of making the same

ABSTRACT

A chip resistor includes a resistor element in the form of a chip, and at least two electrodes formed on the resistor element. The resistor element includes an upper surface, a lower surface, and two end surfaces extending between the upper and the lower surfaces and spaced from each other. The two electrodes are provided on the lower surface of the resistor element. Each of the end surfaces of the resistor element is formed with a conductor film integrally connected to a corresponding one of the electrodes. The conductor film is made of copper, for example, and is higher in solder-wettability than the resistor element.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a chip resistor and method ofmaking the same.

[0003] 2. Description of the Related Art

[0004] An example of prior art chip resistor is shown in FIG. 14 of thepresent application (See JP-A-2002-57009). The illustrated chip resistorR1 includes a resistor element 90 in the form of a chip made of metal,and a pair of electrodes 91 provided on the lower surface 90 b of theresistor element with a gap 93 defined therebetween. Each of theelectrodes 91 has a lower surface formed with a solder layer 92.

[0005] The chip resistor R1 is made by a method as shown in FIG. 15.First, two metal plates 90′ and 91′ as materials for the resistorelement 90 and the electrodes 91 are prepared (a). Subsequently, themetal plate 91′ is laminated and bonded to the lower surface of themetal plate 90′ (b). The metal plate 91′ is then partially removed bymachining to form a gap 93 (c). Thereafter, a solder layer 92′ is formedon the lower surface of the metal plate 91′ (d). Finally, the metalplate 90′, 91′ are cut (e). As a result, a desired chip resistor R1 isobtained.

[0006] Generally, a chip resistor is soldered to a printed circuitboard, for example. In this case, it is preferable that part of thesolder becomes a solder fillet adhering to an end surface of theresistor element of the chip resistor. Such a solder fillet serves tostrongly bond the chip resistor to the circuit board and also to providegood conduction between the chip resistor and the circuit board.Further, by checking the presence or absence of the solder fillet fromoutside, it is possible to determine whether or not the chip resistor isproperly bonded to the circuit board. Specifically, when a solder filletis formed, the mounting of the chip resistor can be determined to beproper. When a solder fillet is not formed, it can be determined thatthe mounting of the chip resistor is probably improper.

[0007] In this way, a solder fillet has many advantages for a chipresistor. However, in the chip resistor R1 shown in FIG. 14, it isdifficult to form a solder fillet on an end surface of the resistorelement 90. This is because the resistor element 90 is made of an alloyto which solder is unlikely to adhere (which has low solder-wettability)such as Ni—Cu alloy, Cu—Mn alloy or Ni—Cr alloy.

SUMMARY OF THE INVENTION

[0008] The present invention is conceived under the circumstancesdescribed above. It is, therefore, an object of the present invention toprovide a chip resistor which enables a solder fillet to be properlyformed in surface-mounting. Another object of the present invention isto provide a manufacturing method which enables efficient manufacturingof such a chip resistor.

[0009] According to a first aspect of the present invention, there isprovided a chip resistor which comprises a chip resistor elementincluding a first main surface, a second main surface opposite to thefirst main surface, a first end surface extending between the first mainsurface and the second main surface, and a second end surface oppositeto the first end surface; and at least two first electrodes provided onthe first main surface and spaced from each other. The first end surfaceis formed with a first conductor film, whereas the second end surface isformed with a second conductor film. The conductor films are higher insolder-wettability than the resistor element.

[0010] With this structure, solder fillets can be properly formed on thefirst end surface and the second end surface of the resistor element insoldering the chip resistor onto a printed circuit board, for example.

[0011] Preferably, each of the first conductor film and the secondconductor film is integrally connected to a respective one of the twofirst electrodes.

[0012] Such a conductor film can be formed by drawing part of theelectrode. With this method, the conductor film can be formedefficiently at a low cost as compared with the formation of theconductor film by plating, for example.

[0013] Preferably, the chip resistor further comprises at least twosecond electrodes provided on the second main surface while being spacedfrom each other to be located opposite to the first electrodes via theresistor element. The first conductor film is integrally connected to arespective one of the two first electrodes, whereas the second conductorfilm is integrally connected to a respective one of the two secondelectrodes.

[0014] Preferably, the first electrodes and the second electrodes aremade of the same material.

[0015] Preferably, the chip resistor further comprises a firstinsulating layer formed on the first main surface of the resistorelement. The first insulating layer is provided between the two firstelectrodes.

[0016] Preferably, the first insulating layer is held in contact withthe two first electrodes.

[0017] Preferably, the chip resistor further comprises a secondinsulating layer formed on the second main surface of the resistorelement. The second insulating layer is provided between the two secondelectrodes.

[0018] Preferably, the chip resistor further comprises a solder layercovering the electrode and the conductor film.

[0019] According to a second aspect of the present invention, there isprovided a method of making a chip resistor. The method comprises thesteps of: preparing a resistor aggregate made of a resistor material andincluding a plurality of electrode layers formed on a main surface ofthe resistor aggregate, the electrode layers being spaced from eachother; dividing the resistor aggregate into a plurality of chipresistors each including a resistor element having a main surface andtwo end surfaces, and at least two electrodes formed on the mainsurface; and forming a conductor film on each of the two end surfaces ofthe resistor element of each of the chip resistors.

[0020] Preferably, the formation of the conductor film is performed bydrawing part of the electrode layer along a division surface of theresistor aggregate in dividing the resistor aggregate.

[0021] Preferably, the division of the resistor aggregate is performedby blanking or cutting.

[0022] Preferably, the formation of the electrode layers is performed byforming an insulating layer into a pattern on the main surface of theresistor aggregate by thick film printing and then metal-plating regionsof the main surface which are not formed with the insulating layer.

[0023] Other features and advantages of the present invention willbecome clearer from the description of the preferred embodiments givenbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a perspective view showing a chip resistor according toa first embodiment of the present invention.

[0025]FIG. 2 is a sectional view taken along lines II-II in FIG. 1.

[0026]FIGS. 3A-3C are perspective views showing part of process stepsfor manufacturing the chip resistor of FIG. 1.

[0027]FIGS. 4A-4B are perspective views showing the process steps to beperformed subsequent to the step of FIG. 3C.

[0028]FIG. 5A is a perspective view showing a resistor aggregate in theform of a bar obtained by the step of FIG. 4B.

[0029]FIG. 5B is a perspective view showing the process step to beperformed subsequent to the step of FIG. 4B.

[0030]FIG. 6 is a perspective view showing an example of method forobtaining individual chip resistors from the resistor aggregate in theform of a plate shown in FIG. 4C.

[0031]FIG. 7 is a perspective view showing a chip resistor according toa second embodiment of the present invention.

[0032]FIG. 8 is a sectional view taken along lines VIII-VIII in FIG. 7.

[0033]FIGS. 9A-9B are perspective views showing part of process stepsfor manufacturing the chip resistor of FIG. 7.

[0034]FIG. 10A is a perspective view showing a resistor aggregate in theform of a bar obtained by the step of FIG. 9B.

[0035]FIG. 10B is a perspective view showing the process step to beperformed subsequent to the step of FIG. 9B.

[0036]FIG. 11 is a perspective view illustrating a chip resistoraccording to a third embodiment of the present invention.

[0037]FIG. 12 is a perspective view illustrating a chip resistoraccording to a fourth embodiment of the present invention.

[0038]FIG. 13 is a perspective view illustrating a chip resistoraccording to a fifth embodiment of the present invention.

[0039]FIG. 14 is a perspective showing a prior art chip resistor.

[0040]FIG. 15 shows a method of making the prior art chip resistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings.

[0042]FIGS. 1 and 2 illustrate a chip resistor according to a firstembodiment of the present invention. The illustrated chip resistor A1includes a resistor element 1, a first insulating layer 2A, a secondinsulating layer 2B and a pair of electrodes 31.

[0043] The resistor element 1 is a chip having an elongated rectangularshape as viewed in plan and a uniform thickness. The resistor element 1has an upper surface 10 a, a lower surface 10 b, and two end surfaces 10c extending between the upper surface and the lower surface. The endsurfaces 10 c are spaced from each other in the longitudinal directionof the resistor element 1. As the material of the resistor element 1,use may be made of Ni—Cu alloy, Cu—Mn alloy or Ni—Cr alloy, for example.The material of the resistor element is not limited to these, and usemay be made of metal materials suitable for the size of the chipresistor A1 and having a resistivity capable of realizing the intendedresistance of the chip resistor.

[0044] The first insulating layer 2A and the second insulating layer 2Bmay be made of epoxy resin by thick film printing, for example. Thefirst insulating layer 2A is provided on the lower surface (first mainsurface) 10 b of the resistor element 1. Specifically, the lower surface10 b includes regions each formed with a respective one of the pairedelectrodes 31 and a remaining region (“non-electrode region”) . Thefirst insulating layer 2A is so provided as to entirely cover thenon-electrode region. The second insulating layer 2B is so provided asto entirely cover the upper surface (second main surface) 10 a of theresistor element 1.

[0045] The paired electrodes 31 are provided on the lower surface 10 bof the resistor element 1 and spaced from each other in the longitudinaldirection of the resistor element 1. The electrodes 31 are made of amaterial (e.g. copper) having a higher solder-wettability than theresistor element 1. As will be described later, the electrodes 31 can beformed by plating the resistor element 1.

[0046] The end surfaces 10 c of the resistor element 1 are each coveredwith a conductor film 31 a. The conductor film 31 a is integrallyconnected to a corresponding one of the electrodes 31. As will bedescribed later, the conductor film 31 a can be formed to extend upwardby drawing part of the copper material constituting the electrode 31.Since copper has high malleability, drawing of copper can provide a filmcovering a wide region.

[0047] As shown in FIG. 2, each of the electrodes 31 has an inner endsurface which is held in contact with an end surface 20 of the firstinsulating layer 2A. With this structure, the distance between the twoelectrodes 31 corresponds to the width s1 of the first insulating layer2A. In the chip resistor A1, the resistance between the pairedelectrodes 31 is set to a relatively low value of about 1 to 100 mΩ.Each of the electrodes 31 has a lower surface on which a solder layer 39is laminated.

[0048] Next, a method of making the chip resistor A1 will be describedwith reference to FIGS. 3-5.

[0049] First, as shown in FIG. 3A, a metal plate (resistor material) 1Ais prepared. The plate 1A has a size (length×width) capable of providinga plurality of resistor elements 1 and has a uniform thickness. Theplate 1A has a first flat main surface 10 a, and a second flat mainsurface 10 b opposite to the first main surface 10 a.

[0050] As shown in FIG. 3B, a second insulating layer 2B′ is formed onthe first main surface 10 a of the plate 1A. The second insulating layer2B′ may be made of epoxy resin by thick film printing, for example.

[0051] Subsequently, as shown in FIG. 3C, a plurality of firstinsulating layers (insulating strips) 2A′ extending in parallel witheach other are formed on the second main surface 10 b of the plate 1A.The first insulating layers 2A′ are formed by thick film printing usingthe same resin and the same apparatus as those used for forming thesecond insulating layer 2B′. By thick film printing, it is possible toform each of the first insulating layers 2A′ precisely into apredetermined size (particularly width).

[0052] As shown in FIG. 4A, conductive layers (electrode layers) 31A′and solder layers 39A′ are formed on the second main surface 10 b of theplate 1A at regions which are not covered with the first insulatinglayers 2A′. The conductive layers 31A′, which are later to be formedinto electrodes 31, may be formed by Cu-plating, for example. By suchplating, it is possible to form the conductive layer 31A′ precisely intoa predetermined dimension without forming a gap between the conductivelayer 31 and the first insulating layer 2A′. The solder layers 39A′ mayalso be formed by plating, for example.

[0053] As shown in FIG. 4B, after the plating, the plate 1A of FIG. 4Ais cut for division along phantom lines C1 extending perpendicularly tothe direction in which the conductive layers 31A′ and the firstinsulating layers 2A′ extend. As a result, resistor aggregates A1′ (Seealso FIG. 5A) in the form of a bar are obtained each including chipresistors A1 connected to each other in series. The pitch of the phantomlines C1 defines the width of the resistor aggregate A1′ (and hence, thewidth of the chip resistors A1). This width corresponds to the width ofthe chip resistors A1 obtained by the subsequent cutting step. Thecutting of the plate 1A may be performed by using a shearing machine ora rotary cutter, for example.

[0054] After the resistor aggregate A1′ as shown in FIG. 5A is formed,the resistor aggregate is cut for division into a plurality of chips, asshown in FIG. 5B. The cutting operation may be performed by punching anexcess portion M between phantom lines C2 downward by using a punch Pnhaving a flat section. By this operation, part of the resistor aggregateA1′ is separated at the excess portion M as a chip resistor A1, whilepart of the conductive layer 31A′ adjacent the excess portion M is drawndownward (along the division surface of the resistor material). Thus,conductor films 31 a covering opposite end surfaces of the chip resistorA1 are provided. By repetitively performing the cutting operation usingthe punch Pn, a plurality of chip resistors A1 are obtained from asingle resistor aggregate A1′.

[0055]FIG. 6 shows a method of making a chip resistor by blanking or diecutting. In this method, blanking is repetitively performed with respectto the plate 1A shown in FIG. 4A to die-cut the plate 1A into aplurality of resistor elements 1 in the form of a chip. In repetitivelyperforming the blanking, a single punch is repetitively used, whereby aplurality of identical chip resistors can be obtained.

[0056] In the blanking operation, the plate 1A is so blanked that eachof two adjacent conductive layers 31A′ together with the relevant solderlayer 39A′ is divided into two parts. By such blanking, electrodes 31and solder layers 39 are formed at opposite ends of the chip resistorelement 1, whereby a chip resistor A1 is obtained. In this case again,since the conductor layers 31A′ are made of copper having highmalleability, part of the conductor layers 31A′ is drawn downward byblanking, whereby conductor films 31 a are easily formed on opposite endsurfaces of the chip resistor 1. The blanking of the plate 1A isperformed with respect to a plurality of blanking regions (spaced fromeach other by a distance S₂), as indicated by phantom lines in FIG. 6.

[0057] Unlike the prior art method, in both of the above-describedmethods, a pair of electrodes are not formed by cutting a metal plate.Therefore, the chip resistor A1 can be manufactured efficiently at arelatively low cost.

[0058] The chip resistor A1 is surface-mounted on an intended mountingobject such as a circuit board by known solder reflow technique.

[0059] The conductor films 31 a are made of copper and have highsolder-wettability. Therefore, in surface-mounting the chip resistor,solder easily adheres also onto the conductor films 31 a, whereby solderfillets Hf are properly formed, as indicated by phantom lines in FIGS. 1and 2. Therefore, whether or not the chip resistor A1 is properlymounted can be determined by checking the presence or absence of thesolder fillets Hf, and the solder bond strength of the chip resistor A1is enhanced. Moreover, due to the presence of the solder fillets Hf, theheat generated when the chip resistor A1 is energized can be transferredto the circuit board through the solder fillets Hf, whereby thetemperature increase of the chip resistor A1 can be suppressed.

[0060] In the chip resistor A1, the non-electrode region at the lowersurface 10 b of the resistor element 1 is covered with the firstinsulating layer 2A. Therefore, in surface-mounting the chip resistorA1, molten solder does not adhere to the lower surface 10 b of theresistor element 1.

[0061] In the chip resistor A1, the dimension s1 between the pairedelectrodes 31 is determined in forming the insulating layer 2A into apattern by thick film printing. In the thick film printing, thedimension of the insulating layer 2A can be set precisely. Therefore,the resistance of the chip resistor A1 can be easily set to an intendedvalue.

[0062]FIGS. 7 and 8 illustrate a chip resistor according to a secondembodiment of the present invention. In these and the subsequentfigures, the elements which are identical or similar to those of thefirst embodiment are designated by the same reference signs as thoseused for the first embodiment.

[0063] In addition to the electrodes (first electrodes) 31 formed on thelower surface 10 b of the resistor element 1, the chip resistor A2 ofthe second embodiment includes a pair of second electrodes 32 spacedfrom each other on the upper surface 10 a of the resistor element 1. Asshown in FIG. 8, the second electrodes 32 face the first electrodes 31via the resistor element 1.

[0064] The chip resistor A2 has longitudinally opposite end surfaces,i.e., an end surface 10 c (right end surface in FIG. 8) formed with aconductor film 31 a and the other end surface 10 c (left end surface inFIG. 8) formed with a conductor film 32 a. The conductor film 31 a isintegrally connected to the first electrode 31 of the right side,whereas the conductor film 32 a is integrally connected to the secondelectrode 32 of the left side. As will be described later, the conductorfilms 31 a, 32 a are formed by drawing part of the copper materialconstituting the first electrode 31 or the second electrode 32.

[0065] According to this chip resistor A2, solder fillets Hf can beformed on the paired second electrodes 32 as well as on the conductorfilms 31 a, 32 a. Therefore, as indicated by phantom lines in FIGS. 7and 8, the solder fillets Hf cover the longitudinally opposite ends ofthe chip resistor A2, whereby the solder bond strength is enhanced. Thisarrangement is also advantageous to suppress the temperature increase ofthe chip resistor in energizing the chip resistor.

[0066] Next, a method of making the chip resistor A2 will be describedwith reference to FIGS. 9 and 10.

[0067] First, as shown in FIG. 9A, an intermediate product for resistoraggregates is prepared. Similarly to the method of making the chipresistor A1, the intermediate product for resistor aggregates isprepared by forming first and second insulating layers 2A′, 2B′ onopposite surfaces of a plate 1A into predetermined patterns and thenforming first and second conductive layers 31A′, 32A′ and solder layers39A′ by plating. Subsequently, as shown in FIG. 9B, the intermediateproduct is cut along phantom lines C3 to provide resistor aggregatesA2′, (shown in FIG. 10A as enlarged).

[0068] After the resistor aggregate A2′ is formed, the resistoraggregate is cut for division into a plurality of chips, as shown inFIG. 10B. Specifically, the resistor aggregate is cut along phantomlines C4 in the figure by using a shearing machine 40, for example. Incutting, part of the first conductive layer 31A′ located on the rightside of the phantom line C4 is drawn downward to form a conductor film31 a on the left end surface of a chip resistor A2. Further, part of thesecond conductive layer 32A′ located on the left side of the phantomline C4 is drawn upward to form a conductor film 32 a on the right endsurface of a chip resistor A2. By repetitively performing such cuttingoperation, chip resistors A4 each provided with conductor films 31 a and32 a are provided.

[0069] In the above embodiment, the conductor films 31 a, 32 a areformed by respectively utilizing first and second electrodes 31, 32formed on upper and lower surfaces of the chip resistor A2. With thismethod, conductor films are formed on both sides of the cut surface by asingle cutting operation using the shearing machine 40. Therefore, ascompared with the cutting operation using a punch, excess portions forcutting need not be provided, whereby the plate 1 can be utilized moreefficiently.

[0070]FIG. 11 shows a chip resistor according to a third embodiment ofthe present invention. Each end of the chip resistor A3 shown in thefigure includes a solder layer 39 covering surfaces of a first electrode31, a second electrode 32 and a conductor film 31 a. With thisstructure, a solder fillet Hf can be easily formed on these surfaces.

[0071]FIG. 12 shows a chip resistor according to a fourth embodiment ofthe present invention. Each end of the chip resistor A4 shown in thefigure includes a conductor film 31 a covering an end surface 10 c ofthe resistor element 1 only partially, not entirely. With this structureagain, a solder fillet Hf can be formed on the end surface 10 c of theresistor element 1.

[0072]FIG. 13 shows a chip resistor according to a fifth embodiment ofthe present invention. Each end of the chip resistor A5 shown in thefigure includes a conductor film 31 a covering an end surface 10 c andpart of paired side surfaces 10 d of the resistor element 1. With thisstructure, it is possible to form a solder fillet Hf spreading in threedirections from the chip resistor A5, as indicated by phantom lines inthe figure.

[0073] Although the conductor film is formed by drawing part of anelectrode in each of the foregoing embodiments, the present invention isnot limited thereto. For instance, the conductor film may be formed byplating or by making a film of a material having high solder-wettabilityand then bonding the film to the resistor element. To enhance theproductivity, it is preferable to make the conductor film from the samematerial as that of the electrodes. However, the conductor film may bemade of a material different from that of the electrodes as long as thematerial of the conductor film is higher in solder-wettability than theresistor element.

[0074] According to the present invention, one chip resistor may beprovided with no less than two pairs of electrodes. When such a largenumber of electrodes are provided, only part of the electrodes may beused, for example.

[0075] In the above-described manufacturing methods, a resistor materialin the form of a plate is first prepared for division into a pluralityof chip resistors. However, the present invention is not limitedthereto. For instance, instead of using a plate-like resistor material,a resistor material in the form of a bar may be used from the first.

[0076] The present invention being thus described, it is apparent thatthe same may be varied in many ways. Such variations should not beregarded as a departure from the spirit and scope of the presentinvention, and all such modifications as would be obvious to thoseskilled in the art are intended to be included within the scope of thefollowing claims.

1. A chip resistor comprising: a chip resistor element including a firstmain surface, a second main surface opposite to the first main surface,a first end surface extending between the first main surface and thesecond main surface, and a second end surface opposite to the first endsurface; and at least two first electrodes provided on the first mainsurface and spaced from each other; wherein the first end surface isformed with a first conductor film, whereas the second end surface isformed with a second conductor film, the conductor films being higher insolder-wettability than the resistor element.
 2. The chip resistoraccording to claim 1, wherein each of the first conductor film and thesecond conductor film is integrally connected to a respective one of thetwo first electrodes.
 3. The chip resistor according to claim 1, furthercomprising at least two second electrodes provided on the second mainsurface and spaced from each other to be located opposite to the firstelectrodes via the resistor element, wherein the first conductor film isintegrally connected to a respective one of the two first electrodes,whereas the second conductor film is integrally connected to arespective one of the two second electrodes.
 4. The chip resistoraccording to claim 3, wherein the first electrodes and the secondelectrodes are made of a same material.
 5. The chip resistor accordingto claim 1, further comprising a first insulating layer formed on thefirst main surface of the resistor element, the first insulating layerbeing provided between the two first electrodes.
 6. The chip resistoraccording to claim 5, wherein the first insulating layer is held incontact with the two first electrodes.
 7. The chip resistor according toclaim 3, further comprising a second insulating layer formed on thesecond main surface of the resistor element, the second insulating layerbeing provided between the two second electrodes.
 8. The chip resistoraccording to claim 1, further comprising a solder layer covering theelectrode and the conductor film.
 9. A method of making a chip resistorcomprising the steps of: preparing a resistor aggregate made of aresistor material and including a plurality of electrode layers formedon a main surface of the resistor aggregate, the electrode layers beingspaced from each other; dividing the resistor aggregate into a pluralityof chip resistors each including a resistor element having a mainsurface and two end surfaces, and at least two electrodes formed on themain surface; and forming a conductor film on each of the two endsurfaces of the resistor element of each of the chip resistors.
 10. Themethod according to claim 9, wherein the formation of the conductor filmis performed by drawing part of each electrode layer along a divisionsurface of the resistor aggregate in dividing the resistor aggregate.11. The method according to claim 10, wherein the division of theresistor aggregate is performed by blanking or cutting.
 12. The methodaccording to claim 9, wherein the formation of the electrode layers isperformed by forming an insulating layer into a pattern on the mainsurface of the resistor aggregate by thick film printing and thenmetal-plating regions of the main surface which are not formed with theinsulating layer.